This invention relates to circuits for electronic strobe lights such as are used to provide visual warning in electronic fire alarm devices and other emergency warning devices and, more particularly, to a control circuit for causing plural strobes connected to the same fire alarm control panel to flash on synchronism with one another.
Strobe lights are used to provide visual warning of potential hazards or to draw attention to an event or activity. An important field of use for strobe lights is in electronic fire alarm systems, frequently in association with audible warning devices, such as horns, to provide an additional means for alerting persons who may be in danger. Strobe alarm circuits include a flashtube and a trigger circuit for initiating firing of the flashtube, with the energy for the flash typically supplied from a capacitor connected in shunt with the flashtube. In some known systems, the flash occurs when the voltage across the flash unit (i.e., the flashtube and associated trigger circuit) exceeds the threshold value required to actuate the trigger circuit, and in others the flash is triggered by a timing circuit. After the flashtube is triggered it becomes conductive and rapidly discharges the stored energy from the shut capacitor until the voltage across the flashtube has decreased to a value at which the flashtube is extinguished and becomes non-conductive.
In a typical installation, a loop of several flash units is connected to a fire alarm control panel which includes a power supply for supplying power to all flash units in the loop when an alarm condition is present. The supply voltage may typically be 12 volts or 20-31 volts, and may be either D.C. supplied by a battery or a full-wave rectified voltage. Underwriters Laboratories specifications require that operation of the device must continue when the supply voltage drops to as much as 80% of nominal value and also when it rises to 110% of nominal value. The power supply typically is provided from first and second terminals which will normally have negative and positive polarity, respectively, when no alarm condition is present, and which reverse when an alarm condition is present, as is usual in supervised systems. When an alarm condition is present, power is supplied to all of the strobe units connected in the loop, with each unit firing independently of the others at a rate determined by its respective charging and triggering circuits and satisfying UL specifications that the flash rate of such visual signalling devices must fall between 20 and 120 flashes per minute.
To counteract claims by epileptic groups that viewing multiple visual signalling devices each flashing at different points in time may trigger a seizure in susceptible individuals. Underwriters Laboratories may additionally require that such signalling systems be controlled in a manner to insure that an individual viewing multiple units could see effective flash rates no higher than 5 flashes per second. Thus, there is a need for controlling multiple self-timed visual signalling devices in a way which will insure that individuals viewing multiple units could see effective flash rates no higher than 5 flashes per second.
It is a primary object of the present invention to provide a circuit having these properties and which also will work with:
(a) both D.C. and full-wave power rectified supplies;
(b) all fire alarm control panels;
(c) mixed strobes (i.e., 110 candela and 15 candela); and
(d) audio as well as visual signalling devices.
Another object of the invention is to provide a circuit having these properties which can be manufactured at relatively low cost.
Another object is to provide a control circuit which will not interfere with the supervision function of the alarm system, and which will be compatible with both constant power and constant current strobe circuits.
Still another object is to provide a control circuit for synchronizing flashing of multiple strobes which, in the event of its failure, will allow each of the individual strobes to flash at its own self-timed rate.
Another object of the invention is to provide such control circuit for synchronizing flashing of multiple strobes and having capability to limit the energy per flash of the associated strobe circuits to that required to meet mandated requirements.
In accordance with the invention, a control circuit is provided which causes multiple strobes connected in a common circuit or loop to flash at the same time, in synchronism, at a rate no higher than a predetermined rate, for example, 5 flashes per second. The control circuit, which may either be incorporated in the fire alarm control panel which controls the loop, or interposed between the fire alarm control panel and the loop of strobes, derives its power from the control panel in the same way as the strobes do: during supervision when the polarity of the power supply is reversed, it uses no power, but when an alarm condition is present it becomes powered and starts operating in a sync: node. When in the sync mode, once every flash cycle, typically at intervals of 2.9 seconds, the control circuit interrupts power to all of the strobes for a period of from 10 to 30 milliseconds, this being the signal which causes all of the strobes in the loop to flash. At the same time, this signal resets the internal timer of each flash unit to ready it for arrival of the next sync signal. In the event no sync signal arrives after an interval exceeding 2.9 seconds, each strobe unit will flash when its flash timer completes its cycle.
The synchronizing control circuit of the invention may be used in conjunction with a variety of strobe circuit designs, preferably having the following desirable properties: (a) an energy limiter operable over a predetermined voltage range in the sync mode; (b) a trigger circuit which is responsive to the sync signals; and (c) a resettable timer for recycling the strobe unit in a non-sync mode in case of lack of the sync signal.